Operators of industrial hoists designed for lifting heavy loads frequently must move the load to a precise position such as is required when a large workpiece is inserted into a lathe or other metal forming tool. In other circumstances it is desireable to gently lower a heavy load from an elevated, suspended position to a rest position in a manner which avoids marring either the suspended load or the support surface. In many instances these types of maneuvers have proven difficult to accomplish and have required extraordinary measures for their satisfactory completion.
In an attempt to solve this positioning problem, many have purchased expensive two speed hoists which have normal and low speeds: the hope being that the low speed would enable more precise positioning of a load. Such is not the case however. Even the low speed operating mode of a two speed hoist does not permit precise load positioning without up and down jogging which itself is not a completely satisfactory solution since actuation of the hoist cannot be precisely controlled by manual actuation so that final position depends of chance.
Another prior approach to the problem has been to insert a micro-positioning device intermediate the hoist and the load as disclosed in U.S. Pat. No. 3,730,484. Such a device, however, reduces the vertical distance that a given hoist can lift a load by an amount equal to the vertical dimension of the device, or requires the hoist to be raised to a higher position to compensate for the vertical distance occupied by insertion of the device intermediate the hoist and load. Furthermore, such devices, whether they operate on pneumatic power, hydraulic power or electrical power, permit the uncontrolled drop of the load by the vertical throw of the device upon loss of such power.
Other approaches have been attempted such as those disclosed in U.S. Pat. Nos. 2,752,120 and 2,801,760 with varying degrees of success. These other approaches, however, require major mechanical components such as fluid couplers and vertically adjustable pulleys and are not easily retrofit to an existing hoist if such retrofit is possible at all. Thus, in most instances, hoist operators have had to cope with a hoist without the assistance of these above mentioned load positioning devices. Experienced hoist operators using a conventional hoist attempt to obtain a precise position by first moving the load to the approximate desired height and then "jogging" the hoist up and down until the load chances to be positioned at the desired height. This "jogging" technique obviously is hard on hoist components, especially the motor relays and contactors and the brake and is entirely unsuitable where the load must be delicately lowered to a fixed rest surface. Inexperienced hoist operators on the other hand are apt to hold the hoist "down" control for too long a period so that the load tends to over run the point of load location thus damaging the part or the machine.